Many electrical circuits include switching elements for controlling operation of the circuit. One such type of circuit, is an inverter circuit adapted for operation at or near a resonant frequency of the circuit. An inverter circuit changes direct current (DC) power to alternating current (AC) power. An illustrative inverter circuit includes first and second switching elements coupled in a half-bridge configuration. The switching elements are controlled to facilitate resonant operation of the circuit. A circuit path to a load energized by the inverter circuit includes resistive, inductive and capacitive circuit elements coupled in various series and parallel arrangements.
Inverter circuits generally have diodes, known as free wheeling or antiparallel diodes, coupled across terminals of the switching elements such that the diodes are biased into a conduction state when the switching element is biased into its non-conduction state and vice-versa. These diodes become conductive for a period of time during which none of the bridge switching elements are conductive. This time is commonly referred to as dead time. During the dead time, the free wheeling diodes provide a discharge path for energy stored in the resonant inductive and capacitive circuit elements. As known to one of ordinary skill in the art, if the charge stored in the circuit elements is not provided with a discharge path, the energy will resonate in parasitic capacitive and inductive circuit elements. Such resonance will generate high voltages and frequencies that could destroy circuit elements.
While the free wheeling diodes provide a discharge path during the dead time, these diodes do not prevent avalanche current flow through the switching elements. The voltage drop across a conductive free wheeling diode can reverse bias a switching element thereby facilitating avalanche current flow through the switching element. The undesirable avalanche current flow can damage the switching element and may reduce the reliability of the switching element and any circuit which includes the switching element. Avalanche current flow will also slow switching element transitions between conductive and non-conductive states due to the time required for the stored energy from the avalanche current to discharge.
It would, therefore, be desirable to provide a circuit having switching elements that are biased to reduce or eliminate avalanche current flow.